1. Field of the Invention
The present invention relates to a physical amount estimating apparatus, a road surface friction condition estimating apparatus, a steering angle neutral point estimating apparatus and a tire air pressure reduction estimating apparatus, and particularly to a physical amount estimating apparatus for estimating a physical amount based on a corrected value with a hysteresis characteristic removed therefrom, a road surface friction condition estimating apparatus for calculating a corrected value with a hysteresis characteristic, caused by influence from viscous friction of Coulomb friction, removed therefrom, using an estimated value of self aligning torque (SAT) and estimating a road surface friction condition based on the calculated corrected value, a steering angle neutral point estimating apparatus for estimating a neutral point of a steering angle based on a corrected value with a hysteresis characteristic removed therefrom and a steering angle, and an air pressure reduction estimating apparatus of a tire for estimating a reduction in air pressure of a tire based on a corrected value with a hysteresis characteristic removed therefrom and an SAT reference value.
2. Description of the Related Art
Japanese Patent Application Laid-open (JP-A) No. 11-287749 discloses technology for detecting a steering angle and steering torque of a tire, calculating a characteristic of the steering torque with regard to the steering angle and calculating a friction coefficient xcexc (road surface xcexc) of a road surface on which a tire is grounded.
According to the related art, the road surface xcexc, which constitutes a physical amount in correspondence with a grip state, is calculated as an amount of change of the steering torque with regard to an amount of change of the steering angle, and therefore the resulting estimating method is liable to be affected by noise. That is, calculation of an amount of change signifies carrying out differentiation, which amplifies noise, and therefore the estimated value becomes a value that includes a large amount of noise.
Further, according to the related art, the road surface xcexc is estimated only when the steering angle is increased in order to steer. Thus, at a maximum steering angle, at which a load applied to the tire is maximized, the steering angle cannot be increased in order to steer, and therefore the friction coefficient cannot be estimated. Inherently, if the road surface xcexc is estimated when the load applied to the tire is maximized, that is, near a limit, or in other words, when limit road surface xcexc is estimated, accuracy of the estimation is promoted. According to the related art, however, there is a problem in that the road surface xcexc cannot be estimated at the maximum steering angle and the road surface xcexc can be estimated only before reaching the maximum steering angle.
Meanwhile, a hysteresis characteristic is caused by twisting of a tire tread, Coulomb friction in a power steering apparatus, or the like, between SAT and a slip angle or a steering angle. Therefore, the characteristic differs when increasing the steering angle and when decreasing the steering angle, and according to the related art, in which attention is paid to a change in SAT with regard to a change in the slip angle or the steering angle, a dispersion in the estimated value is caused.
Further, when a driver reduces steering torque to the point of not moving the steering wheel when steering is maintained, although there is no change in the slip angle or the steering angle, as a result of reducing SAT, there is a possibility of erroneously determining that a xe2x80x9creduction in gripxe2x80x9d has occurred. That is, according to the above-described related art, although such an erroneous determination is avoided and the dispersion in the estimated value is reduced by carrying out the estimation only when the steering angle is increased, as a result, there is a problem in that a road surface friction condition of the grip state or the like cannot be estimated when decreasing or maintaining the steering angle.
Further, that the road surface friction condition cannot be estimated when decreasing or maintaining the steering angle, signifies that when the tire shifts from a low xcexc road to a high xcexc road while maintaining the steering angle, or when the grip state is changed by shifting from a high xcexc road to a low xcexc road, at the time of the change, the road surface friction condition and the grip state cannot be estimated until the steering angle is subsequently increased. Therefore, the estimated value of the grip state according to the related art cannot be utilized as a control parameter for switching a characteristic of a power steering apparatus or ABS, which require swift adaptability in accordance with the grip state.
Further, JP-A Nos. 11-334634 and 11-59466, disclose technology in which reference steering torque, which is set based on a steering angle and vehicle speed, is compared with steering torque, and when a state in which the steering torque is greater than the reference steering torque continues for a certain period of time, it is determined that a reduction in air pressure of a tire has occurred.
In the above-described related art, the steering torque, which includes friction of a steering system, is utilized, and therefore there is a problem in that a change in the steering torque with regard to air pressure is influenced by the friction. As a result, the change cannot accurately be detected and a reduction in air pressure cannot be estimated with high accuracy due to the influence of the friction.
Further, the relationship between the steering angle and the steering torque is also influenced by movement of the vehicle, which is a dynamic characteristic, aside from the friction of the steering system. Therefore, there is a concern that the accuracy of estimation is deteriorated when fast steering is carried out.
As a countermeasure against the above-described problem, according to the related art, a condition is added that the state in which the steering torque is greater than the reference steering torque must continue for a certain period of time or longer, and due to the condition, the problem of deterioration in accuracy is alleviated. As a result of the condition, however, chances to carry out estimation are decreased, and there is a new problem in that estimation time is delayed.
The invention has been carried out in order to resolve the above-described problem and it is an object thereof to provide a physical amount estimating apparatus capable of estimating a highly accurate physical amount by removing a hysteresis characteristic such that a dispersion is not caused in an estimated value, a road surface friction condition estimating apparatus capable of accurately estimating a road surface friction condition without steering to increase a steering angle, a steering angle neutral point estimating apparatus capable of accurately estimating a neutral point of a steering angle and an air pressure reduction estimating apparatus of tire capable of estimating reduction in the pressure of tire.
In order to achieve the above-described object, a physical amount estimating apparatus of the invention is constituted by including first outputting means for outputting a first physical amount which is not provided with a hysteresis characteristic, second outputting means for outputting a second physical amount which has a predetermined physical relationship with the first physical amount and has a hysteresis characteristic, hysteresis removing means for calculating a corrected value removed of the hysteresis characteristic based on the second physical amount, and estimating means for estimating a third physical amount based on a physical relationship between the first physical amount and the corrected value.
According to the invention, the hysteresis characteristic is removed based on the physical amount having the predetermined physical relationship with the first physical amount which is not provided with the hysteresis characteristic and having the hysteresis characteristic, other physical amount is estimated based on the physical amount removed of the hysteresis characteristic and therefore, the highly accurate physical amount can be estimated. Thereby, still other physical amount can be estimated from the highly accurate physical amount and therefore, the dispersion is not caused in the estimated physical amount.
According to the invention, it is possible that the first physical amount is constituted by a physical amount with regard to a steering state of a steered wheel, for example, either one of a slip angle of the steered wheel and a side force of the steered wheel, the second physical amount is constituted by an estimated value of a self aligning torque estimated from a steering torque and an assist torque, the corrected value is constituted by a corrected value of the self aligning torque calculated from the estimated value of the self aligning torque and the third physical amount is constituted by a physical amount with regard to either one of a wheel state in running a vehicle and a road state.
It is possible that the corrected value of the self aligning torque is calculated by a calculating equations, which each include a gradient represented by a ratio of a change in the corrected value of the self aligning torque to a change in the estimated value of the self aligning torque, and which equations are different for respective regions due to a gradient in a region, where the estimated value of the self aligning torque varies due to Coulomb friction, being made smaller than gradients of regions other than the region.
Further, the hysteresis characteristic can be removed by representing a predetermined physical relationship between the physical amount having the hysteresis characteristic and the above-described same physical amount which is not provided with the hysteresis characteristic by a graph, a table or the like and converting the physical amount having the hysteresis characteristic into the physical amount which is not provided with the hysteresis characteristic based on the physical relationship. In this case, conversion is facilitated by previously determining an initial value in accordance with previous history.
The physical amount with regard to either of the wheel state in running the vehicle and the road state (for example, air pressure reduction of tire, or road surface friction condition) can be estimated based on either one of the slip angle of the steered wheel and the side force of the steered wheel and the corrected value of the self aligning torque, or based on a reference value of the self aligning torque set in accordance with either one of the slip angle of the steered wheel and the side force of the steered wheel, and the corrected value of the self aligning torque.
Further, the physical amount with regard to either one of the wheel state in running the vehicle and the road state can be estimated also based on a gradient of the corrected value of the self aligning torque represented by the ratio of the change of the corrected value of the self aligning torque to the change of the slip angle, the gradient of the corrected value of the self aligning torque represented by the ratio of the change of the corrected value of the self aligning torque to the change of the side force.
Further, the physical amount with regard to either one of the wheel state in running the vehicle and the road state can also be estimated based on the physical amount with regard to a steering state of a steered wheel and either one of a reference value of the slip angle and the corrected value of the self aligning torque.
The reference value of the slip angle may be calculated by a calculating equations, which each include a gradient represented by a ratio of a change in the reference value of the slip angle to a change in the estimated value of the self aligning torque, and which equations are different for respective regions due to a gradient in a region, where the estimated value of the self aligning torque varies due to Coulomb friction, being made smaller than gradients of regions other than the region.
A road surface friction condition estimating apparatus according to the invention is constituted by including a steering angle sensor for detecting a steering angle, a vehicle speed sensor for detecting a vehicle speed, a torque sensor for detecting a steering torque, an assist torque sensor for detecting an assist torque of steering, slip angle estimating means for estimating a slip angle of a steered wheel based on the steering angle and the vehicle speed, self aligning torque estimating means for estimating a self aligning torque based on the steering torque and the assist torque, hysteresis removing means for calculating a corrected value removed of a hysteresis characteristic based on an estimated value of the self aligning torque, and friction state estimating means for estimating a road surface friction condition from the corrected value and the slip angle.
The slip angle estimating means of the road surface friction condition estimating apparatus of the invention, estimates the slip angle of the steered wheel (for example, front wheel) by using the steering angle and the vehicle speed based on a vehicle motion model. The self aligning torque (SAT) estimating means estimates SAT constituting road reaction force based on the steering torque and the assist torque, specifically by adding the steering torque and the assist torque. The steering torque is detected by, for example, a torque sensor attached coaxially with a steering wheel and the assist torque is calculated, for example, from current of the electric power steering apparatus.
Further, accuracy of estimating SAT can be promoted also by subtracting viscous friction torque produced in proportion to steering angular velocity from a value added with the steering torque and the assist torque. Further, SAT may be estimated by a method of a disturbance observer described in Japanese Patent Application No. 2000-370704 by utilizing also the steering angular velocity other than the steering angle. By using the method of the disturbance observer, there can be estimated SAT in consideration of a torque produced by inertia of the power steering apparatus.
The hysteresis removing means outputs the corrected value removed of the hysteresis characteristic based on the estimated value of the self aligning torque.
That is, the hysteresis removing means can calculate, as the corrected value, the slip angle with regard to the estimated value of the self aligning torque based on the physical relationship between the self aligning torque having the hysteresis characteristic and the slip angle. That is, there is outputted, as the corrected value, the slip angle (slip angle reference value) in consideration of the hysteresis characteristic produced by Coulomb friction of the power steering apparatus from the estimated value of SAT estimated by the SAT estimating means. The reference value of the slip angle is, for example, a slip angle of the steered wheel, for example, front wheel produced in a steering region having a sufficiently high grip state and allowance and therefore, the hysteresis characteristic with regard to the estimated value of the self aligning torque is removed.
The friction state estimating means estimates the grip state, that is, the road surface friction condition by comparing the slip angle of the steered wheel estimated by the slip angle estimating means and the reference value of the slip angle outputted from the hysteresis removing means and utilizing the fact that the more reduced is the grip state, that is, the road surface friction condition, the larger the slip angle becomes than the reference value of the slip angle.
Further, the hysteresis removing means may calculate the self aligning torque removed of the hysteresis characteristic from the estimated value of the self aligning torque as the corrected value of the self aligning torque based on the physical relationship between the estimated value of the self aligning torque and the corrected value of the self aligning torque removed of the hysteresis characteristic in place of the above-described removing method. In this case, the self aligning torque removed of the hysteresis characteristic (corrected value of SAT) can directly be calculated from the estimated value of the self aligning torque.
The friction state estimating means estimates the grip state, that is, the road surface friction condition by comparing the reference value of SAT calculated by multiplying the slip angle of the steered wheel estimated by the slip angle estimating means by a coefficient which is changed in accordance with vehicle speed or kind of tire and the corrected value of SAT and utilizing the fact that the more reduced is the grip state, that is, the road surface friction condition, the smaller the corrected value of SAT becomes than the reference value of SAT.
As has been explained above, according to the invention, there is outputted the corrected value removed of the hysteresis characteristic based on the self aligning torque, the road surface friction condition is estimated based on the corrected value and therefore, road surface friction condition can be estimated accurately in all of a range of the steering angle.
Further, a steering angle neutral point estimating means according to the invention is constituted by including a steering angle sensor for detecting a steering angle, a torque sensor for detecting a steering torque, an assist torque sensor for detecting an assist torque of steering, self aligning torque estimating means for estimating a self aligning torque based on the steering torque and the assist torque, hysteresis removing means for calculating a corrected value removed of the hysteresis characteristic constituting an initial value by a value when an estimated value of the self aligning torque becomes 0 based on the estimated value of the self aligning torque, and neutral point estimating means for estimating the steering angle when the corrected value becomes 0 as a neutral point of the steering angle based on the corrected value and the steering angle.
The self aligning torque (SAT) estimating means according to the invention estimates SAT based on the steering torque and the assist torque as explained above. The hysteresis removing means outputs the corrected value removed of the hysteresis characteristic from the physical relationship between SAT having the hysteresis characteristic and SAT removed of the hysteresis characteristic by constituting an initial value by a value when the estimated value of SAT become 0. Further, the neutral point estimating means estimates the steering angle when the corrected value becomes 0 as the neutral point of the steering angle based on the corrected value and the steering angle.
An air pressure reduction estimating apparatus of tire according to the invention is constituted by including a steering angle sensor for detecting a steering angle, a vehicle speed sensor for detecting a vehicle speed, a torque sensor for detecting a steering torque, an assist torque sensor for detecting an assist torque of steering, slip angle estimating means for estimating a slip angle of a steered wheel based on the steering angle and the vehicle speed, self aligning torque estimating means for estimating a self aligning torque based on the steering torque and the assist torque, hysteresis removing means for calculating a corrected value of the self aligning torque removed of a hysteresis characteristic based on an estimated value of the self aligning torque, and air pressure reduction estimating means for estimating whether an air pressure of a tire is reduced based on the corrected value of the self aligning torque and the slip angle.
According to the air pressure reduction estimating apparatus of tire, there may be estimated whether the air pressure of tire is reduced by providing a lateral acceleration sensor for detecting lateral acceleration and a yaw angular velocity sensor for detecting yaw angular velocity in place of the steering angle sensor and the vehicle speed sensor, for estimating a side force of the steered wheel based on the lateral acceleration and the yaw angular velocity and based on the corrected value of the self aligning torque and the side force.
The reduction in the air pressure of tire can be estimated based on the corrected value of the self aligning torque and a reference value of the self aligning torque set in accordance with the slip angle of the steered wheel, based on the corrected value of the self aligning torque and the reference value of the self aligning torque set in accordance with the side force of the steered wheel, or based on a gradient of the corrected value of the self aligning torque represented by a ratio of a change of the corrected value of the self aligning torque to a change of the slip angle.
Further, it may be estimated whether the air pressure of tire is reduced based on a gradient of the corrected value of the self aligning torque represented by a ratio of a change of the corrected value of the self aligning torque to the change of the side force.
The ratio of the reference value of SAT to the corrected value of SAT calculated by the air pressure reduction determining means, is a value which is increased when the air pressure is reduced and determination of air pressure reduction can be carried out when the value exceeds a threshold. However, the value is simultaneously a value representing the grip state and becomes a small value when the grip is reduced as in running on the low xcexc road. Therefore, when the ratio of the reference value of SAT to the corrected value of SAT exceeds the threshold of determining the air pressure reduction, it can be determined that the air pressure is reduced and when the ratio of the reference value of SAT to the corrected value of SAT becomes lower than the threshold of determining reduction of the grip, it can be determined that the grip is reduced.
Further, whereas the reduction of the ratio of the reference value of SAT to the corrected value of SAT caused by the reduction of grip is brought about in accordance with steering and the change is comparatively fast, the increase of the ratio of the reference value of SAT to the corrected value of SAT caused by air pressure reduction, is slow in the change. Therefore, when the ratio of the reference value of SAT to the corrected value of SAT is identified by an on-line least squares method, in contrast to a logic for estimating the grip degree, according to a logic of estimating air pressure, it is preferable that the slow change of the ratio of the reference value of SAT to the corrected value of SAT is caught by setting a large forgetting factor or thinning data referred to as decimation. Further, when air pressure reduction is determined, it is preferable to use data at a vicinity of a straight advancement state having the small slip angle and not influenced by the reduction of grip and therefore, it is also conceivable to remove, by selection, data in carrying out comparatively large steering having large slip angle.
As has been explained above, according to the invention, the hysteresis characteristic is removed from the second physical amount having the hysteresis characteristic, the third physical amount is estimated based on the second physical amount removed of the hysteresis characteristic and therefore, there is achieved an effect of capable of estimating the highly accurate physical amount.
Further, the corrected value removed of the hysteresis characteristic is outputted based on the self aligning torque, the road surface friction condition or air pressure reduction of tire is estimated based on the corrected value and therefore, there is achieved an effect of capable of estimating the road surface friction condition or air pressure reduction of tire accurately in all the range of the steering angle.
Further, the corrected value removed of the hysteresis characteristic is outputted based on the self aligning torque, the steering angle neutral point is estimated based on the corrected value and therefore, there is achieved and effect of capable of accurately estimating the steering angle neutral point.